The present invention relates to a method of producing a frame unit for a machine for comminuting or granulating plastics, such as a granulator mill or a shredder, the frame unit being provided at least with anchorages for a granulator mill housing and a drive motor, and the frame unit being at least partly cast from a plastic- or concrete-based casting material in a mold which, in the finished state of the frame unit, has surfaces for cooperation with the mill housing.
The present invention also relates to a frame unit produced according to the method, and a machine for comminuting or granulating plastics material using such a frame unit.
In machines for comminuting plastic, and in particular such plastic as is to be recycled and reused, there are extremely exacting requirements in place that the comminuted plastic must be completely free of impurities such as dust, foreign matter, metal fragments, gravel etc. The reason is that such impurities, even in extremely slight quantities, would render the comminuted plastic material completely unusable. As a result, extremely stringent requirements are in place relating to cleanliness in, at and around all of the machinery handling the plastic material, naturally also including the machinery that is employed for the comminution or granulation proper.
For the sake of simplicity, the discussion below will refer to granulator mills for the plastics industry, without, to that end, other types of machinery for comminuting plastics being excluded, for instance shredders.
Granulators of the type that are employed in the plastics industry for recycling plastic materials generate enormous amounts of noise and vibration. Particularly sever conditions arise if the granulator is fed with large lumps of hard plastics material which are thrown about inside the housing of the granulator mill during the initial phase of a grinding operation.
Those requirements on precision that apply to granulators of the type under consideration here are extremely stringent, since there are disposed, interiorly in the granulator mill housing, a number of fixed knives which cooperate with corresponding knives on a rotor rotating in the granulator mill housing. The clearance between the fixed and moving knives while in operation is extremely slight, and must be kept constant with extremely narrow tolerances even if the granulator mill is supplied with large lumps of plastics material.
If the above-described precision and the rigidity in the granulator mill housing and the rotor could not be guaranteed, the risk is imminent that the knives would clash with one another, so that metal fragments would become admixed in the finished plastic granulate.
Traditionally, granulator mills have been constructed on a supporting frame system consisting of or comprising welded steel profiles of heavy-duty dimensions, partly for reasons of mechanical strength and partly for the reason that the intention is to impart to the granulator mill as great a mass as possible in order to avoid vibrations and noise being transmitted to the ambient environment.
The mill housing proper is generally manufactured as a self-supporting unit, which, as was mentioned above, houses on the one hand the rotor and on the other hand the fixed knives. Traditionally, such a granulator mill housing is bolted in place to the above-mentioned framework system which also serves as a mounting for the drive motor which drives the mill. Further components are normally also disposed on the framework, such as for instance auxiliary aids for opening and closing the infeed section of the mill, conduits for finished granulate or conveyors for such material as is to be ground. The mill housing has feet which are machined to extremely stringent tolerances, which also applies to the surfaces of the framework system on which the feet of the mill housing are bolted in place.
It is previously known in the art to employ machine beds or foundations consisting of or comprising concrete. The main purpose of such foundations is to create the mass which is needed for the machine in question to stand still. Another reason may also be to insulate a machine in terms of vibration from the surroundings with a very high degree of accuracy, so that ground vibrations, for example from nearby traffic, do not reach the machine. Such foundations need not maintain any particularly narrow tolerances, since the machine in question is often provided with adjustment devices which can compensate for any possible deviations and shortcomings.
It is also previously known in the art to manufacture bearing machine components, such as frame units or sections etc., from different types of casting materials, both plastic-based and concrete-based. One example of such a construction is shown in EP 0293322 A2.
Concrete possesses the property that it may emit dust and small particles even without any manifest surface damage, and smaller or larger chips or chunks if it is subjected to powerful mechanical action, for example if a heavy object is dropped on it or if a truck is driven over it.
It is desirable to design the method intimated by way of introduction such that a frame unit may be realised which possesses the requisite large mass at low cost. It is also desirable to design the method such that the precision in the finished frame unit will be sufficiently great so as not to require any retro-working before the granulator mill is mounted in place. It is also desirable to design the method such that the risk of contamination of comminuted or granulated plastic material is obviated as far as is possible.
According to an aspect of the present invention, the method intimated by way of introduction is characterised in that the mold, after the casting operation, is left in position on the casting material, that the mold is given surfaces for cooperation with the granulator mill housing and that the surfaces of the mold intended for cooperation with the mill housing are kept fixed within predetermined tolerances during the casting operation.